Xeroderma pigmentosum (XP) is a human disease attributed to defective DNA repair. Afflicted individuals suffer from acute sun sensitivity and a predisposition to skin cancer. Fibroblasts from these patients have been studied in an effort to understand human DNA repair mechanisms and processes which predispose to cancer. Studies with these cells define XP into nine genetic complementation groups which exhibit characteristic phenotypes of hypersensitivity to UV radiation and reduced repair synthesis. Although such phenotypes appear to offer amenability to complementation by DNA transfer, little progress has been made in the quantitation of DNA complementation, chromosome mapping of the genes involved, or cloning of these genes. The present proposal offers an alternative approach to address each of these objectives in XP cells. An existing collection of rodent/human hybrids each bearing a characterized, single human chromosome which is tagged with a biochemically selectable marker, will be used for the microcell transfer of individual human chromosomes to various XP complementation group. Complementation in the absence of any rodent DNA transfer permits assignment of the complementing factor to the single human chromosome present in the donating hybrid. Complementation will be evaluated quantitatively for both UV sensitivity and repair synthesis. For selected XP group(s), the generation of hybrids bearing a series of complementing and non- complementing deletions of this human chromosome will yield finer mapping and provide construction useful for the identification of human DNA sequences unique to the complementing hybrids. These sequences will serve as probes for cDNA or genomic libraries to identify the complementing gene. Successful cloning of a single human repair gene will provide an established approach for cloning gene involved in other XP groups, and the deletion hybrids should prove useful to the scientific community for the mapping and cloning of additional human genes for which more classical approaches have also failed.